U.S. patent application number 10/014580 was filed with the patent office on 2002-09-12 for plasma torch provided with a ceramic protective cap.
Invention is credited to Augeraud, Regis, Delzenne, Michel.
Application Number | 20020125220 10/014580 |
Document ID | / |
Family ID | 8847765 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020125220 |
Kind Code |
A1 |
Augeraud, Regis ; et
al. |
September 12, 2002 |
Plasma torch provided with a ceramic protective cap
Abstract
Protective cap for a plasma arc working torch, includes a cap
body (7) having an opening (17) for the passage of plasma gas, the
cap body (7) being made of at least one material of ceramic type.
The ceramic is a silicon nitride or an aluminum silicate.
Preferably, the ceramic cap body (7) is clad with boron nitride
deposited on the external surface of the cap body, the thickness of
the cladding of boron nitride being less than 3 mm. Torch provided
with such a cap and its use in a steel plate plasma cutting
operation.
Inventors: |
Augeraud, Regis; (Lesquin,
FR) ; Delzenne, Michel; (Franconville, FR) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
8847765 |
Appl. No.: |
10/014580 |
Filed: |
December 14, 2001 |
Current U.S.
Class: |
219/121.5 ;
219/121.48 |
Current CPC
Class: |
H05H 1/34 20130101; H05H
1/3473 20210501 |
Class at
Publication: |
219/121.5 ;
219/121.48 |
International
Class: |
B23K 010/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2000 |
FR |
0002852 |
Claims
1. Protective cap for a plasma arc working torch, comprising a cap
body (7) comprising an opening (17) for passage of plasma gas, at
least one portion of said cap body (7) being comprised of at least
one ceramic type material, characterized in that said ceramic is
aluminum silicate.
2. Protective cap for a plasma arc working torch comprising a cap
body (7) comprising an opening (17) for passage of plasma gas, at
least one portion of said cap body (7) being comprised of at least
one material of ceramic type, characterized in that said ceramic is
silicon nitride having one or several of the following properties:
a density of 2 to 4 kg/dm.sup.3, a thermal shock resistance greater
than or equal to 600.degree. C., a volumetric resistivity of at
least 10.sup.10 .OMEGA..cm, a coefficient of thermal expansion less
than 6.10.sup.-6/.degree. C., a flexure resistance at 20.degree. C.
of at least 200 MPa, a compressive resistance of at least 650
MPa.
3. Cap according to claim 1 or 2, characterized in that said cap
body (7) has at least one cylindrical cross-section and an axis of
revolution coaxial with said opening (17), preferably said cap body
(7) has the general shape of a sleeve or a cup.
4. Cap according to one of claims 1 to 3, characterized in that
said ceramic cap body (7) is at least partially clad with boron
nitride deposited on the external surface of the ceramic cap body,
preferably the thickness of the boron nitride coating is less than
3 mm.
5. Cap according to one of claims 1, 3 or 4, characterized in that
the ceramic is aluminum silicate formed of SiO.sub.2,
Al.sub.2O.sub.3 and one or several additional constituents selected
from TiO.sub.2, Fe.sub.2O.sub.3, CaO, MgO, K.sub.2O, Na.sub.2O and
P.sub.2O.sub.5, in the following proportions: at least 60% by
weight of SiO.sub.2, at least 25% by weight of Al.sub.2O.sub.3 and
the rest being essentially one or several additional constituents
selected from TiO.sub.2, Fe.sub.2O.sub.3, CaO, MgO, KO, Na.sub.2O
and P.sub.2O.sub.5, preferably the ceramic is aluminum silicate
formed of 60 to 80% by weight SiO.sub.2, from 25 to 35% by weight
of Al.sub.2O.sub.3 and the rest (up to about 100% by weight) one or
several additional constituents selected from TiO.sub.2,
Fe.sub.2O.sub.3, CaO, MgO, K.sub.2O, Na.sub.2O and
P.sub.2O.sub.5.
6. Cap according to one of claims 1 to 5, characterized in that
said cap body (7) is formed by a downstream portion (7a) at least
partially of ceramic having the opening (17), fixed to an upstream
portion (7b) serving as a support permitting securing said cap body
(7) to a plasma torch, preferably the downstream portion (7a) in
ceramic is fixed to the upstream portion 97b) by clinching or
cementing and said upstream portion (7b) is of metal or metallic
alloy.
7. Cap according to one of claims 1 to 6, characterized in that the
external wall of the cap body (7) comprises one or several external
protuberances (15), preferably said protuberances (15) have a
dimension comprised between 0.5 and 2 mm relative to the surface of
the cap (7).
8. Plasma torch comprising a torch body (12) and a torch head (13),
provided with an electrode and at least one nozzle (2),
characterized in that it comprises a protective cap (7, 7a, 7b, 17)
according to one of claims 1 to 7 surrounding at least in part said
nozzle (2) so as to form a mechanical, electrical and thermal
barrier about said nozzle (2).
9. Torch according to claim 8, characterized in that the torch body
(12) and the torch head (13) are removable from each other.
10. Automatic machine, in particular for plasma cutting, comprising
a torch according to claim 8 or 9.
11. The use of a torch according to one of claims 8 or 9 or of a
machine according to claim 10, to cut at least one metallic plate
by using a plasma jet, preferably a steel plate.
Description
[0001] The present invention relates to a protective cap for the
nozzle of a plasma arc torch and a plasma torch provided with such
a cap.
[0002] A plasma torch usable in the cutting, welding, marking,
projection or any other operation for thermal treatment of a
metallic or non-metallic material, conventionally comprises a
copper or copper alloy electrode carrying a cylindrical insert
generally of hafnium, tungsten or zirconium, on which the electric
arc can take root which serves to ionize the gas supplying the
torch, which is to say the predetermined flow rate of gas under
pressure, so-called plasmagenic gas, which is distributed between
the electrode and the nozzle and which flows through an orifice of
said nozzle in the direction of the workpiece.
[0003] The electrode is generally centered above this ejection
opening of the plasma jet arranged axially in the nozzle and which
forms a constriction diaphragm.
[0004] In the particular case of a plasma cutting operation, the
plasma cutting device or system conventionally comprises a plasma
torch, a source of electric current, a system for striking the
electric arc and one or several sources of fluids, in particular
plasmagenic gas, if desired a protective gas or post-injected
fluid, and a cooling fluid for the torch, generally distilled
water.
[0005] Such torches or installations are well known to those
skilled in the art because they have already been described in
numerous documents which can be referred to for greater detail,
particularly EP-A-599709, EP-A-872300, EP-A-801882, EP-A-941018,
EP-A-144267, EP-A-410875, EP-A-772957, EP-A-902606, EP-A-810052,
EP-A-845929, EP-A-790756, EP-A-196612, WO-A-89/11941, U.S. Pat. No.
4,521,666, U.S. Pat. No. 4,059,743, U.S. Pat. No. 4,163,891 and
U.S. Pat. No. 5,591,357.
[0006] In known manner, plasma arc cutting uses the thermal and
kinetic effects of a plasma jet to melt the material to be cut and
to expel the melted material from the kerf formed consecutive to
relative displacement of the torch and the workpiece; the
composition or nature of the plasmagenic gas used varies according
to the nature of the material to be cut.
[0007] The torch nozzle generally has an intermediate electric
potential comprised between the electrical potential of the cathode
and that of the plate to be worked.
[0008] However, for mechanical reasons or for errors of operation,
it can happen that the torch comes into contact with the plate.
[0009] There is thus created a parasitic arc which instantly
destroys the nozzle, which thus translates into a very substantial
deterioration of the quality of the cut and/or requires the
operator to change the nozzle.
[0010] The duration of use of the nozzle is thus reduced, which
increases considerably the cost of the process because the nozzles
used must be replaced more frequently by new nozzles.
[0011] So as to overcome this problem, it has already been proposed
to arrange a protective cap about the nozzle of the torch so as to
form an insulating mechanical barrier between the nozzle and the
plate so as to protect the nozzle, during possible contact with the
plate.
[0012] To do this, certain documents provide for the use of
stumatite caps, which is a natural ceramic, or alumina.
[0013] However, it has been noted in practice that if these
materials resist thermal shock, they are relatively fragile and can
break easily in case of shock against a plate.
[0014] Moreover, it is also known to use a zirconia cap.
[0015] However, here again, it has been noted on an industrial
scale that, if zirconia caps were more durable than stumatite caps
or alumina caps, they have, on the other hand, a low resistance to
shock and thermal stress.
[0016] The problem which accordingly arises is to have a cap for a
plasma torch constituted by an insulating material permitting
resisting not only high temperature of the plasma jet, which is to
say thermal stresses, but also mechanical stresses, particularly
shocks against the plate to be worked.
[0017] The object of the present invention is thus to solve these
problems by providing a protective cap for plasma torch formed of a
material having high resilience to resist mechanical shocks, a low
coefficient of expansion to resist thermal shocks and which will
moreover be a good electrical insulator.
[0018] Preferably, the protective cap according to the invention
must also be relatively smooth, which is to say without roughness,
such that the flag projected during working the metal will not
adhere or will adhere the least possible to the cap.
[0019] The present invention thus relates to a protective cap for a
plasma arc working torch, comprising a cap body comprising an
opening for passage of plasma gas, said cap body being comprised of
at least one material of ceramic type, characterized in that said
ceramic is silicon nitride or aluminum silicate, preferably silicon
nitride.
[0020] As the case may be, the cap of the invention can comprise
one or several of the following characteristics:
[0021] said cap body has at least one cylindrical section and an
axis of revolution coaxial with said opening, preferably said cap
body has the general shape of a sleeve or cup.
[0022] the ceramic cap body is at least partially clad with boron
nitride deposited on the external surface of the cap body,
preferably the thickness of the boron nitride coating is less than
3 mm. The deposit of the layer of boron nitride on the external
surface of the cap body is carried out, for example, by sputtering
of boron nitride.
[0023] the ceramic is aluminum silicate formed of SiO.sub.2,
Al.sub.2O.sub.3 and of one or several added constituents selected
from TiO.sub.2, Fe.sub.2O.sub.3, CaO, MgO, K.sub.2O, Na.sub.2O and
P.sub.2O.sub.5, and if desired unavailable impurities, in the
following proportions: at least 60% by weight of SiO.sub.2, at
least 25% by weight of Al.sub.2O.sub.3 and the rest being
essentially one or several of the added constituents selected from
TiO.sub.2, Fe.sub.2O.sub.3, CaO, MgO, K.sub.2O, Na.sub.2O and
P.sub.2O.sub.5. Preferably, the ceramic is aluminum silicate formed
by 60 to 80% by weight SiO.sub.2, preferably 60 to 70% of
SiO.sub.2, from 25 to 35% by weight of Al.sub.2O.sub.3, preferably
from 28 to 34% by weight of Al.sub.2O.sub.3, and for the rest (up
to about 100% by weight) one or several additional constituents
selected from TiO.sub.2, Fe.sub.2O.sub.3, CaO, MgO, K.sub.2O,
Na.sub.2O and P.sub.2O.sub.5.
[0024] the ceramic is silicon nitride having one or several of the
following properties: a density of 2 to 4 kg/dm.sup.3, preferably
2.3 to 3.5 kg/dm.sup.3; a resistance to thermal shock greater than
or equal to 600.degree. C.; a volume resistivity of at least
10.sup.10 .OMEGA..cm, preferably at least 10.sup.12 .OMEGA..cm; a
coefficient of thermal expansion less than 6.10.sup.-6/.degree. C.,
preferably less than 4.10.sup.-6/.degree. C.; a flectural
resistance at 20.degree. C. of at least 200 MPa, preferably at
least 600 MPa; and/or a compressive strength of at least 650 MPa,
preferably at least 1500 MPa.
[0025] the body of the cap has a thickness comprised between 2 mm
and 10 mm, at the level of or adjacent the opening for passage of
the plasma gas.
[0026] the body of the cap comprises securement means permitting
fixing or arranging securely the cap on a plasma torch about at
least one portion of the nozzle of said torch, so as to surround
and protect said nozzle.
[0027] the cap body is formed of a downstream portion at least
partially of ceramic having the orifice, fixed to an upstream
portion serving as a support permitting securing said cap body on
the plasma torch, preferably of the downstream ceramic portion
being fixed to the upstream portion by crimping, cementing or any
other suitable technique and said upstream portion is of a metal or
a metal alloy, for example brass.
[0028] the external wall of the cap body comprises one or several
external protuberances, preferably said protuberances have a
dimension comprised between 0.5 and 2 mm relative to the surface of
the cap.
[0029] The invention also relates to a plasma torch comprising a
torch body and a torch head provided with an electrode and at least
one nozzle, characterized in that it comprises a protective cap
according to the invention surrounding at least a portion of said
nozzle so as to form an effective mechanical, electrical and
thermal barrier about said nozzle.
[0030] As the case may be, the torch body and the torch head are
separable from each other, as explained in EP0 599 709, to which
reference may be had for further details.
[0031] Moreover, the invention also relates to an automatic
machine, in particular for plasma cutting, comprising a torch
according to the invention, as well as to the use of a torch or of
a machine according to the invention to cut at least one metal
plate by the use of a plasma jet, preferably a steel plate.
[0032] Thus, the inventors of the present invention have discovered
that, among many ceramics adapted to be used as a material for
making a cap for a plasma torch, namely the silicates comprising
the steatites and the cordierites, titanium oxides, aluminas and
zirconias, the natural ceramics, such as stumatite, certain among
them having particularly unexpected properties, which is to say
that they have a high resiliency to resist mechanical shocks, a low
coefficient of thermal expansion to resist thermal shocks and can
be rendered relatively smooth by simple machining.
[0033] These particular ceramics are silicon nitride (KERSIT.TM. or
KERNIT.TM.) and aluminum silicate, which ceramics permit
harmonizing all of the above-mentioned requirements.
[0034] A protective cap for a plasma torch comprised by one or the
other of the particular ceramics according to the present
invention, is preferably dimensioned so as to cover at most the
nozzle of the torch whilst not interfering with the operation of
this latter, which is to say not disturbing the passage of the
plasma gas stream.
[0035] Thus the distance between the plate and the nozzle being
very small, the thickness of the cap must be as small as possible
and must permit evacuation of the slag above the plate.
[0036] Moreover, protuberances on the cap permit holding the cap
spaced from the plate without interfering with the evacuation of
slag or of the gas.
[0037] Moreover, preferably, an external coating on the cap
constituted by boron nitride permits, because of its
physical-chemical properties, avoiding slag or molted metal from
splashing from the plate during piercing of the latter, for
example, not becoming stuck to the protective cap clad with this
external cladding.
[0038] The invention will now be better understood from the
accompanying drawings and from the examples, given by way of
illustration but not limitation, of the invention.
[0039] FIG. 1 shows schematically a plasma coating torch supplied
by an electric current source 5 connected to an electric arc
striking system 6 permitting generating a pilot arc between the
nozzle 2 and the electrode 1, and by various fluids, such as
plasmagenic gas 3. An insulating protective cap 7 of ceramic
according to the invention protects the nozzle 2 of the torch.
According to a particular embodiment, the insulating cap of ceramic
can be assembled by cementing, crimping or any other assembly
means, to a metallic support of copper, brass or any other metallic
material. During cutting, a plasma jet melts the material 11 to be
cut and expels the molten material 10 from the kerf formed
consecutive to relative movement of the torch and the workpiece 11.
The plasma torch also comprises an electrode 1 from which the root
of the electric arc proceeds, serving to ionize the gas supplying
the torch, which is to say the predetermined stream of plasmagenic
gas under pressure, which is distributed and flows in the plasma
chamber located between the electrode 1 and the torch 2. The plasma
jet is then expelled from the plasma chamber by an opening provided
in the nozzle 2 and in the direction of the plate 11 to be cut. To
do this, the electrode 1 is conventionally centered above the
ejection opening of the jet plasma arranged axially in the torch
2.
[0040] A cap according to the invention can be provided for any
type of plasma torch, no matter whether it is in one part or in two
parts separable from each other (schematically shown in FIG. 2),
namely a torch head 13 and a torch body 12, such as the
disassembleable torch described in EP0 599 709, as well as shown in
FIGS. 3 and 4 which show the head 13 of the torch after separation
of the body 12 from the torch.
[0041] As can be seen in FIGS. 3 or 4, the protective cap 7
according to the invention can be formed in two parts, namely:
[0042] a downstream part 7a of ceramic comprising the passage
opening for the gas and adapted to surround the major portion of
the torch 2, and
[0043] an upstream portion 7b in metal, such as brass, serving as a
support and metallic frame, and serving for the connection and
securement of the cap on the torch 1.
[0044] In the embodiment of FIG. 3, the upstream ceramic portion is
crimped on the metallic support, whilst in the embodiment of FIG.
4, the upstream ceramic portion is cemented to said metallic
support.
[0045] FIG. 5 shows a protective cap according to the invention in
profile so as to show the protuberances 15 on the downstream
portion 7a of the body 7 of the cap which permit avoiding any
mechanical contact with the plate without interfering with
evacuation of the slag or the gas between the torch provided with
the cap and the plate to be worked on.
[0046] FIG. 6 shows a bottom view of the cap of FIG. 5 better
showing the arrangement of the protuberances 15, which are three in
number and distributed about the outlet opening 17.
[0047] As can be seen in FIG. 7, which is a longitudinal
cross-section on the line A-A of the cap of FIGS. 5 and 6, the
protective cap for a plasma torch according to the invention is of
cylindrical-truncated conical shape and is comprised, in this
embodiment, of a particular ceramic 11 according to the present
invention, fixed, for example by cementing, to an internal metallic
structure or frame 10.
[0048] The cap body 7 also comprises securement means 18, such as a
screw thread or the like, permitting securing or arranging securely
the cap on a plasma torch about at least a portion of the nozzle of
said torch, so as to surround and protect said nozzle.
[0049] Preferably, a layer of cladding of boron nitride is disposed
along the ceramic portion and is renewed after a predetermined time
of use of the torch or after wear of this latter.
[0050] The protective cap 7 according to the invention can be used
to protect the nozzle and the other fragile pieces of the head of
the torch of no matter what type of plasma cutting installation,
whether manual or automatic.
[0051] Preferably, the torch according to the invention is of the
monoflux type, the gas flow being adapted to be rotary or
axial.
[0052] The invention has been described above with respect to the
plasma cutting torch, but of course the application of this
invention is not limited only to cutting torches and relates in
whole or in part to marking, welding, projection torches and
generally any torch for thermal treatment of metallic or
non-metallic materials.
Example of an Aluminum Silicate Cap According to the Invention
[0053] Table I hereafter gives the chemical composition of an
aluminum silicate usable to produce all or a portion of a
protective cap for a plasma torch according to the present
invention.
1 TABLE 1 Proportion of Elements constituting the each element
aluminum silicate (% by weight) Ignition loss (drying at 0.61
110.degree. C.) SiO.sub.2 63.58 Al.sub.2O.sub.3 31.44 TiO.sub.2
1.79 Fe.sub.2O.sub.3 1.02 CaO 0.11 MgO <0.08 K.sub.2O 0.81
Na.sub.2O 0.23 P.sub.2O.sub.5 0.18 TOTAL about 99.77
[0054] The values (%) are determined by determining the weight
ratio in grams of each constituent per 100 grams of material
(aluminum silicate) dried at 110.degree. C. and are determined by
x-ray spectrometry.
Example of a Silicon Nitride Cap According to the Invention
[0055] The following Table II gives the characteristics of several
silicon nitrides (NS1, NS2, NS3) usable to produce all or a part of
a protective cap for a plasma torch according to the present
invention.
2 TABLE II Properties NS1 NS2 NS3 Density 2.5 3.3 3.2 (in
kg/dm.sup.3) Thermal shock 200 850 650 resistance (variation of
temperature in .degree. C.) Volume >10.sup.10 >10.sup.12
>10.sup.12 resistivity (in .OMEGA..cm) Coefficient 3.1 3.3 3.3
of thermal expansion (10.sup.-6/.degree. C.) Flexure >600
>600 >600 resistance at 20.degree. C. (in MPa) Compressive
650 >3000 >3000 resistance (in MPa)
[0056] The silicon nitrides (NS1, NS2, NS3) having the above
characteristics are available commercially, particularly from the
MORGAN-MATROC company under the marks RBSN, HPSN and SSN,
respectively, or else from the NORTON-DEMARQUEST company under the
marks KERSIT.TM. or KERNIT.TM..
[0057] The silicon nitrides NS2 and NS3 are preferred because they
have thermal and mechanical resistance properties and insulating
properties, that are better than those of NS1.
* * * * *